As a vehicle-mounted visual auxiliary system, a head-up display (HUD) is capable of projecting vehicle state information (e.g., speed and fuel quantity) and indication information (e.g., navigation information and warning information) to an appropriate position in front of a driver, so as to enable the driver to know the vehicle state information and the indication information with his sight line not departing from a road ahead. In this way, it is able to prevent the occurrence of a blind zone when the driver is looking at a dashboard, thereby to improve driving security and safety.
In order to enable the information displayed by the HUD to be viewed clearly by the driver, it is necessary to adjust a display brightness value of the HUD in such a manner that a contrast between the display brightness value of the HUD and an ambient brightness value is within an appropriate range. Currently, a method for adjusting the display brightness value of the HUD includes detecting the ambient brightness value through a brightness sensor at a bottom of a windshield of a vehicle or at a vehicle body, and then adjusting the display brightness value of the HUD in accordance with the detected ambient brightness value, so as to enable the contrast between the display brightness value of the HUD and the ambient brightness value to be within the appropriate range. Because the brightness sensor is arranged at the bottom of the windshield or at the vehicle body, the above method merely detects the ambient brightness value at the bottom of the windshield or the vehicle body. Usually, a virtual image is to be formed by the HUD at a region in front of the vehicle. In this case, it is impossible for the ambient brightness value detected by the brightness sensor to actually reflect an ambient brightness value at an imaging position of the HUD. At this time, the contrast between the display brightness value of the HUD and the ambient brightness value is not within the appropriate range, and the adjustment accuracy of the display brightness value is relatively low.